In the field of painting/coating, to obtain a coating viscosity suitable for a coating step such as coating and printing, a solvent type coating composition to be diluted with a solvent has been widely used heretofore. However, the use of an organic solvent has problems in aspects of safety and health as well as working environment. In the case of a solvent-free type coating composition, a step of drying the solvent is not required, however, a step of heating of the coating composition is sometimes required to obtain a coating viscosity. This presented problems such as skin toxicity and odor of a diluting monomer to be added, similar to the solvent type coating composition. Moreover, a polymer material cannot be used because of limitation of the coating viscosity, and therefore chemical and physical performances of the coating film cannot be easily improved.
For the coating composition for coating metals used to protect surfaces of molding metal materials and metal plates, not only safety and environmental compatibility, but also generally required performances such as hardness of coating film, high adhesion and solvent resistance specific to the field of metal coating, and performances which make it possible to weld directly without removing a protective coating film formed in a welding step in post-processing, are required.
To solve these problems, intensive study and development about an aqueous coating composition have recently been made. Dispersion or dissolution of a resin in an aqueous medium is attained by using a dispersant or an emulsifier, or an action of a dispersible group, emulsifiable group or soluble group localized internally (ionic functional group or a nonionic functional group), and the latter self-emulsifiable or self-soluble resin has secured comparatively excellent performances. However, since it is required to introduce sufficient water-soluble functional groups to attain self-emulsion in a molecule, good balance between the water resistance and the emulsifiability is necessary, resulting in poor physical properties of the coating film such as boiling water resistance and solvent resistance.
Japanese Unexamined Patent Application, First Publication, No. Sho 61-83262, Japanese Examined Patent Application, Second Publication, No. Hei 2-41555, and Japanese Unexamined Patent Application, First Publication, No. 5-320568 disclose a method of adding a reaction product of an epoxy resin and phosphoric acid or a reaction product of an epoxy resin, a phosphate and carboxylic acid into an aqueous acrylic resin and an aqueous amino resin as the method of improving the adhesion of a thermosetting composition. Although the resulting thermosetting composition has excellent adhesion, a thin coating film having a thickness of several microns does not satisfy the solvent resistance, adhesion and workability. Various methods have been suggested, but there has never been suggested a thin coating film having a thickness of several microns formed on the surface of a metal material, particularly a metal sheet, which has excellent adhesion, durability to severe working and excellent resistance to a solvent, particularly a polar solvent.
Recently, a radiation curable aqueous coating composition has been intensively studied as an environmentally friendly coating composition using no organic solvent. As a resin, which is a basic component constituting a coating film-forming component, for example, a polyurethane resin, an acrylic resin, a polyester resin and an epoxy resin are known. The acrylic resin has an excellent chemical resistance because of a carbon-carbon bond on a principal chain, but cannot easily attain high workability. The polyester resin can attain excellent workability because of a flexible ester bond on a principal chain, but has poor chemical resistance such as acid resistance or alkali resistance. The epoxy resin has excellent adhesion and excellent chemical resistance, but cannot easily attain good balance between high workability and solvent resistance. Thus, these resins have good points and bad points. The polyurethane resin has a drawback that a urethane bond and a hydrogen bond are liable to be formed by the reaction of isocyanate as a raw material, thus resulting in high viscosity. However, since good balance between the functionality and performance can be easily attained by utilizing high reactivity of an isocyanate group and the urethane bond exists in a molecule, molecules entangle with each other via the hydrogen bonds and serve as a pseudo-polymer, and therefore, the resulting coating film is superior in adhesion.
For example, an invention described in Japanese Unexamined Patent Application, First Publication No. Hei 8-259888 provides an aqueous dispersion comprising water and radiation curable microgel particles dispersed in water as a polyurethane resin-based radiation curable aqueous coating composition having excellent coating film-forming property. The microgel particles are gel-like coating film-forming polyurethane resin particles comprising polyurethane resins and a crosslinked structure obtained by crosslinking polyurethane resins via a urethane bond or a urea bond, and said resin particles have a radiation curable ethylenic unsaturated double bond and a group of a salt. In the above Publication, it is described that the coating film-forming polyurethane resin particles can contain 1 to 50% of the other radiation curable compound and an initiator for radiation curing reaction therein, and that an aqueous coating film having excellent workability and excellent solvent resistance can be obtained by using these radiation curable microgel particles.
Japanese Unexamined Patent Application, First Publication No. Hei 9-31150 discloses, as a composition capable of forming a highly crosslinked coating film, a radiation curable aqueous polyurethane resin composition prepared by reacting a product, which is obtained by the Michael addition of a hydroxyl compound having an active hydrogen to a double bond of a compound having two or more (meth)acryloyl groups, a polyhydroxy compound having an acidic group and a polyisocyanate as an indispensable component. Generally, a composition having an acryloyl group, which is superior in curability, has conventionally been used as a radiation curable composition, and Japanese Unexamined Patent Application, First Publication No. Hei 10-251360 discloses that a radiation curable aqueous polyurethane composition having an methacryloyl group is superior in adhesion, chemical resistance, surface hardness and yellowing resistance, and is used in coating compositions, coating agents and inks.
In Japanese Unexamined Patent Application, First Publication No. Hei 10-251361, it is described that excellent adhesion, excellent solvent resistance, excellent workability and excellent coatability can be obtained by mixing radiation curable microgel particles with a radiation curable aqueous polyurethane resin.
These radiation curable aqueous polyurethane resin compositions are used in coating compositions, coating agents and inks because of excellent adhesion to any organic material (e.g. plastic films, wood construction materials and decorative papers for construction materials) and inorganic materials (e.g. metal materials and glass) and excellent solvent resistance. Any of them is a radiation curable aqueous composition containing an aqueous polyurethane resin having both a radiation curable ethylenic unsaturated double bond and a group of a salt, and the coating film thereof is cured by a radiation and the solvent resistance is enhanced as the acryloyl group concentration and crosslinking degree increase. However, there is such a drawback that the adhesion and the workability are deteriorated as the crosslinking degree increases. With respect to the solvent resistance, a group of a salt and a polar group must be introduced to obtain an aqueous resin composition, resulting in high polarity of the chemical composition itself. Even if the crosslinking degree of the composition is considerably increased, it is difficult to obtain the resistance to a polar solvent, particularly ethanol.
If the crosslinking degree is increased to obtain the solvent resistance, it is impossible to cope with a more severe situation, for example, application where high workability in drawing of a metal plate as well as adhesion are required.
Japanese Unexamined Patent Application, First Publication No. Hei 10-298213 suggests, as a composition having good stability of an emulsion and good adhesion to a base material, a radiation curable emulsion containing a radiation curable emulsifier which has a polymerizable double bond and also has an anionic hydrophilic group such as carboxyl group, phosphate group, carboxylic acid salt group, phosphate salt group or the like. However, the resulting emulsion type composition is not homogenized at a molecular level on formation of a coating film and localization of the component inevitably occurs, thus resulting in poor physical properties such as surface smoothness and gloss of the coating film as well as poor solvent resistance and poor chemical resistance. Since Newtonian flow does not occur, the resulting coating composition is inferior in coatability during roll coating as compared to a water-soluble type coating composition.
In the case in which a coated metal material is welded, the electrical resistance increases and the voltage drop becomes severe when a coating film has a large thickness at the joint portion. Therefore, an applied voltage required to welding must be enhanced. Even when using a conventional coating composition, a problem of the voltage drop is solved by reducing the thickness of the coating film to several microns or less. On the other hand, the solvent resistance of the coating film and the adhesion to the surface of the metal material are drastically deteriorated, thus resulting in poor workability in the post-processing of the coated metal material.
Although various measures have hitherto been suggested to improve the drawbacks described above, there has never been obtained a radiation curable aqueous coating composition which can form a coating film having both excellent adhesion and excellent solvent resistance in the case in which a thin film having a thickness of several microns is formed on the surface of a metal material using the coating composition. Also no coated metal material was known which was capable of being welded at the joint portion of the surface of a metal material in the state of having a thin coating film having both excellent adhesion and excellent solvent resistance.